Visually Complete aims to indicate what a customer will perceive as the point in time when a site looked complete.
It’s calculated by recording a video of a page loading, which is then broken down into a series of screenshot frames. An image diffing algorithm compares and marks visual completion at the frame with the least amount of difference to the final frame.
Visually complete isn't a faultless metric—sites featuring animating carousels or auto-playing video content can make automated calculation extremely difficult. In these circumstances, it may be better to rely on First Meaningful Paint or Time to Interactive as they output from the browser directly.